Use exact positioning for the ItemList::get_tooltip method.

[RyanStein]

This is a potential resolution for #15241, which makes all ItemLists use the exact position flag on get_item_at_position() for use within get_tooltip(), solving any potential issue with widely inaccurate, technically "closest" items being returned.

Note that this affects all ItemLists, not just the FileSystemDock class which uses it. I imagine the ramifications of this are not too terrible, but it has to be said.

[akien-mga]

Thanks a lot for your contribution!
We are now entering a strict release freeze for Godot 3.0 and will only consider major bug fixes. We won't merge new features and enhancements anymore until 3.0 is released.

Moving this PR to the 3.1 milestone, to be reviewed once the release freeze is lifted. It could eventually be cherry-picked for a future 3.0.1 maintenance release if it doesn't change the user-facing APIs and doesn't compromise the engine's stability.